The present invention relates to a high linearity differential circuit, comprising a DC current source feeding from a supply voltage reference terminal the emitter-collector paths of a differential pair of input transistors, the collector of each of these input transistors being fed from another supply voltage terminal, the base receiving an input voltage signal and the emitter being connected to the DC current source via an emitter load resistor which has the value R, practically identical for each of the input transistors.
A differential circuit of this type forms the basis of the greater part of the input stages of prior-art differential amplifiers in which an output signal of such a stage may be tapped either from the emitter load resistors or also from paired impedances inserted in the connection between the collectors of the transistors of the differential pair and said other supply terminal.
One particular object of the invention is to provide an integrable differential circuit whose gain, which is close to unity (or less than that), is well defined and reproducible, and in which the linearity of the output signal relative to the input signal is satisfied to better than 1%. A circuit having a differential voltage gain equal to 1 is known from the article entitled: "A 10-b 75-MSPS Subranging A/D Converter with Integrated Sample and Hold"--Reinhard Petschacher et al.--published in IEEE Journal of Solid-State Circuits, Vol. 25, No. 6, December 1990. This circuit has a linearity compensation as a result of a forward biased junction inserted in the connection from the collector of each of the input transistors to the supply voltage terminal.
The known circuit has the disadvantage that the output amplitude is considerably limited.
A differential circuit having little gain and a high linearity is sought in analog signal measuring means and, for example, in analog-to-digital converters.
Another prior-art way of obtaining the desired result consists of utilizing an operational amplifier to which substantial feedback is applied so as to determine the desired gain, but this solution is not entirely satisfactory because of the limitation of the pass-band and/or the power consumption and also from the point of view of the complexity of the circuit, thus of the surface area the semiconductor requires and, consequently, of its relatively high price.